Multiply Strings

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Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

#include<stdio.h>#include<stdlib.h>#include<string.h>char *multiply(char *num1, char *num2) {    int i, j, k, n, m;    int flag1 = 0, flag2 = 0, data1, data2;    for(i = 0; num1[i] != '\0'; i++);    n = i;    for(j = 0; num2[j] != '\0'; j++);    m = j;    printf("m:%d, n:%d\n", m, n);    char *res = (char *)malloc(sizeof(char) * (m + n));    for(k = 0; k < (m + n); k++) res[k] = '0';    for(j = m - 1; j >= 0; j--) {        for(i = n - 1, flag1 = 0, flag2 = 0, k = j + n ; i >= 0; i--) {            data1 = (num1[i] - '0') * (num2[j] - '0') + flag1;            if(data1 > 9) {                flag1= data1 / 10;                data1 = data1 % 10;            }            else flag1 = 0;            data2 = (res[k] - '0') + data1 + flag2;            if(data2 > 9) {                flag2= 1;                data2 = data2 % 10;            }            else flag2 = 0;            res[k] = data2 + '0';            k--;        }        res[k] = flag1 + '0';        k--;    }    data2 = res[0] - '0' + flag2;     res[0] = data2 + '0';    if(res[0] == '0') {        for(i = 0; res[i + 1] != '\0'; i++) {            res[i] = res[i + 1];        }        res[i] = '\0';    }    return res;      }void main() {    char *num1 = "123456789", *num2 = "123456789";    char *data = multiply(num1, num2);    printf("%s\n", data);}

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